How can high-resolution representation of the regional seas and aerosols modify regional climate change?

How can high-resolution representation of the regional seas and aerosols modify regional climate change? A fully-coupled regional climate system approach to question current experimental protocol Samuel Somot, F. Sevault, P. Nabat, C. Dubois, Kiki Météo-France / CNRM samuel.somot@meteo.fr

Potential sources of added-value in RCM projection runs Med- domain Better representation of the turbulence due to higher resolution Better representation of the constant Surface Boundary Conditions (topography, land-sea contrast) Better representation of the time-varying Forcings (sea surface temperature, aerosol load, land composition) 2

Potential sources of added-value in RCM projection runs Med- domain Better representation of the turbulence due to higher resolution Better representation of the constant Surface Boundary Conditions (topography, land-sea contrast) Better representation of the time-varying Forcings (sea surface temperature, aerosol load, land composition) But today the standard protocol in RCM projection simulations (e.g. PRUDENCE, ENSEMBLES, ) High-resolution -RCM (with high-resolution constant SBCs) is driven by 1. low-resolution LBCs from GCM runs (1-3 ) 2. low-resolution SST from GCM runs (1-2 ) 3. low-resolution Aerosol forcing data from GCM runs (or even no aerosol varying forcing) 3 4. no land composition change

Objectives and methods Explore protocols for RCM projection simulations that includes new potential sources of added-value Twin simulations using the same RCM and the same driving GCM Given a large-scale climate change signal, how is modifed the regional/local signal? RCP8.5, 2071-2100 vs reference period, Summer season Comparison with the standard -only RCM run Comparison with the CMIP5 driving GCM Evaluate the impact of high-resolution representation of the regional SST forcing (Mediterranean Sea) Using a fully-coupled atmosphere-ocean-river RCM Evaluate the impact of high-resolution representation of the regional aerosol forcing (African dust, European sulfate) Using a fully-coupled atmosphere-aerosol RCM 4

Simulation protocols GCM: CNRM-CM5 Low- Resolution Low- Resolution Aerosols Low-Resolution Ocean Atm, SST, Aero RCM: CNRM-ALADIN52 High-resolution - Low-Resolution ocean SST - Low-Resolution aerosol forcing 5

Simulation protocols GCM: CNRM-CM5 Low- Resolution Low- Resolution Aerosols Low-Resolution Ocean Atm, SST, Aero Atm, Aero RCM: CNRM-ALADIN52 High-resolution - Low-Resolution ocean SST - Low-Resolution aerosol forcing RCM: CNRM-RCSM4 High-resolution High-Resolution Ocean - High-Resolution ocean SST - Low-Resolution aerosol forcing 6

7 Simulation protocols GCM: CNRM-CM5 Low- Resolution Low-Resolution Ocean Atm, SST RCM: CNRM-RCSM5 Highresolution Low- Resolution Aerosols HR-AER High- Resolution Aerosols - Low-Resolution ocean SST - High-Resolution aerosol forcing Atm, SST, Aero Atm, Aero RCM: CNRM-ALADIN52 High-resolution - Low-Resolution ocean SST - Low-Resolution aerosol forcing RCM: CNRM-RCSM4 High-resolution High-Resolution Ocean - High-Resolution ocean SST - Low-Resolution aerosol forcing

Simulation protocols - Historical runs (HIST): 1950-2005 (, ) or 1971-2000 (HR-AER) - Scenario runs (RCP8.5): 2006-2100 (, ) or 2071-2100 (HR-AER) - Spin-up (SPIN) : 2 years (, HR-AER), 130 random years from 1949-1979 () time SPIN 1950 1971 2000 2071 2100 HIST RCP8.5 HR-AER HR-AER ALADIN-Climate, 8 version 5.2, Med- ICRC- CNRM-RCSM4, 2016, Med- Stockholm Colin et al. 2010 ; Herrmann et al. 2011 Sevault et al. 2014 CNRM-RCSM5: Nabat et al. 2015

Summer climate change signal in the run Large-scale climate change signal is consistent in the GCM and RCMs runs Summer precipitation increase in Central Mediterranean (, CNRM-CM5) T2m ( C) Prec (mm/d) MSLP (hpa) Climate change signal in the run in Summer (RCP8.5, 2071-2100 vs 1976-2005, JJA) 9

Impact of the high-resolution SST representation SST warms less in run than in run Spatial pattern of the warming is different (Balearic Sea, Adriatic Sea) Climate change signal in Summer (RCP8.5, JJA, 2071-2100 vs 1976-2005) SST ( C) SST ( C) * Here 10 means the GCM ocean model ICRC- and 2016, Stockholm means the high-resolution Mediterranean Sea model

Impact of the high-resolution SST representation SST warms less in run than in run Spatial pattern of the warming is different (Balearic Sea, Adriatic Sea) Climate change signal in Summer (RCP8.5, JJA, 2071-2100 vs 1976-2005) SST ( C) SST ( C) Difference of climate change signal between and in Summer (2071-2100 vs 1976-2005, RCP8.5, JJA) SST ( C) = - * Here 11 means the GCM ocean model ICRC- and 2016, Stockholm means the high-resolution Mediterranean Sea model

Impact of the high-resolution SST representation T2m climate change signal is modified over sea and surrounding lands Enhanced drying over sea and surrounding lands in T2m ( C) T2m ( C) T2m ( C) Prec (mm/d) Prec (mm/d) Prec (mm/d) Climate change signal in Summer 12 (RCP8.5, 2071-2100 vs 1976-2005, JJA) Difference of Summer climate change signal between and

Impact of the high-resolution SST representation T2m climate change signal is modified over sea and surrounding lands Enhanced drying over sea and surrounding lands in Land-sea contrast and dynamical modifications explain the West Europe signal SST ( C) T2m ( C) Prec (mm/d) MSLP (hpa) Difference of climate change signal between and in Summer (2071-2100 vs 1976-2005, RCP8.5, JJA) 13

Impact of the high-resolution SST representation Contrary to the surface, deep layer heat content increases more in SST signal difference is the surface signature of a difference in the vertical distribution of the heat storage by the Mediterranean Sea SST anomaly ( C) Total heat content anomaly ( C) 1970 2100 1970 2100 Temperature profile anomaly ( C, 2071-2100 vs 1976-2005) 14 West Med East Med

Impact of the high-resolution SST representation The high-resolution sea model has a deeper, stronger and more realistic Mediterranean ThermoHaline Circulation (MTHC) in present-climate than the low-resolution ocean GCM The MTHC in is more efficient to redistribute heat over the vertical Mixed layer depth (m, DJF, 1976-2005) Yearly maximum MLD (m) Adriatic Zonal overturning streamfunction (Sv, 1976-2005) G. of Lions Aegean 15 * Here means the GCM ocean model and means the high-resolution Mediterranean Sea model 1970 2100

16 Simulation protocols GCM: CNRM-CM5 Low- Resolution Low-Resolution Ocean Atm, SST RCM: CNRM-RCSM5 Highresolution Low- Resolution Aerosols HR-AER High- Resolution Aerosols - Low-Resolution ocean SST - High-Resolution aerosol forcing Atm, SST, Aero Atm, Aero RCM: CNRM-ALADIN52 High-resolution - Low-Resolution ocean SST - Low-Resolution aerosol forcing RCM: CNRM-RCSM4 High-resolution High-Resolution Ocean - High-Resolution ocean SST - Low-Resolution aerosol forcing

Impact of the high-resolution aerosols representation AOD (Aerosol Optical Depth) decreases in both runs, but less in HR-AER Over North Africa, AOD can even increase in HR-AER HR-AER shows small-scale pattern in the AOD signal (Po valley, mountain, cities) AOD (-) France : -0.28 Tunisia : -0.14 AOD (-) HR-AER France : +18% Tunisia : +57% Climate change signal (RCP8.5, 2071-2100 vs 1971-2000, JJA) 17 France : -0.23 Tunisia : -0.06 Difference of climate change signal between HR-AER and (2071-2100 vs 1971-2000, RCP8.5, JJA)

Impact of the high-resolution aerosols representation The aerosol forcing modifications lead to climate change signal modifications for the surface shortwave and surface temperature (North Africa, Po valley, Eastern Europe) AOD (-) SWsurf-down (W/m2) T2m ( C) France : +18% Tunisia : +57% France : +32% Tunisia : -240% France : +9% Tunisia : -11% Difference of climate change signal between HR-AER and (2071-2100 vs 1971-2000, RCP8.5, JJA) Climate change signal modifications mostly due to aerosol forcing modifications 18

Impact of the high-resolution aerosols representation Dynamical effects (cloud, advection) also contribute for Western and South-East Europe AOD (-) SWsurf-down (W/m2) T2m ( C) MSLP (hpa) Cloud (%) Difference of climate change signal between HR-AER and (2071-2100 vs 1971-2000, RCP8.5, JJA) 19

Impact of the high-resolution aerosols representation Over North Africa, AOD forcing modification in HR-AER is due to : an increase in dust AOD a reduced decrease in anthropogenic AOD AOD-total AOD-Dust AOD-Anthropo Tunisia: -0.14 no-change Tunisia: 0 Tunisia: -0.14 HR-AER Tunisia: -0.06 Tunisia: +0.02 Tunisia: -0.08 Climate change signal (RCP8.5, 2071-2100 vs 1971-2000, JJA) 20 * Here means the aerosol data coming from the GCM run

Impact of the high-resolution aerosols representation The increase in HR-AER dust AOD due to emission increase Dust emission increase due to an increased frequency of windy days in Africa Emission change (kg.m-2.s-1) Wind speed change (km/h) All aerosols Emission decrease for Europe (-97%) Emission increase for Africa (+60%) Sea salt Desert Dust Sulfate Change in the number of windy days (wind speed 1 m/s) Climate change signal in HR-AER 21 (RCP8.5, 2071-2100 vs 1971-2000, JJA)

Conclusions Sensitivity studies using one RCM forced by a given GCM allow to investigate the impact of regional forcing representation on regional to local climate change signal Better representation of the Mediterranean Sea (MTHC, heat storage vertical distribution) can lead to different SST evolution (reduced warming) with impacts on the regional climate change (reduced or enhanced warming, enhanced drying) Better representation of the Mediterranean aerosols (emission process) can lead to different AOD evolution (increased dust) with impacts on the regional climate change (sign change in surface shortwave, reduced or enhanced warming) Physical understanding of the results increases our confidence. Direct and indirect (dynamical, cloud) effects of the forcing modifications contribute to explain the results Results must be verified in multi-model exercises 22 (see proposed FPS in Med-)

Open issues for samuel.somot@meteo.fr The potential forcing roles of the regional SST evolution and of the regional aerosol evolution are underestimated in the current standard protocol RCM added-value can come from other elements than turbulence and topography Regional/Local climate change may be driven by specific regional climate drivers (SST, aerosols), not only by the interactions between the large-scale drivers and the surface boundary conditions To be investigated through dedicated regionally-designed FPS Discussion about the use of coupled RCM in Thursday, 5-7pm, room U36 in the Geoscience building 23

Summer climate change signal in the run Large-scale climate change signal is consistent in the GCM and RCMs runs T2m ( C) GCM (CNRM-CM5, 150km) Climate change signal in the and CMIP5 runs in Summer (RCP8.5, 2071-2100 vs 1976-2005, JJA) 24